1. Field of the Invention
This invention relates to an electric starter motor provided with a planetary reduction assembly for reducing an rpm of a motor output shaft formed integrally with an armature to transmit to a starter output shaft, and more particularly to a shock absorber mechanism for an electric starter motor for absorbing a shock stress caused by an excessive rotational torque on a loaded side.
2. Description of the Related Art
FIG. 4 is a partial cross-sectional view showing a conventional electric starter motor incorporating a planetary speed reduction device, and FIG. 5 is a frontal view showing a primary part showing a coupled condition of an internal gear of the planetary reduction assembly in this conventional electric starter motor.
In FIGS. 4 and 5, the electric starter motor is composed of a starter motor 3 for generating a rotational torque, a planetary reduction assembly 5 for reducing and outputting an rpm of a motor output shaft 4 of this starter motor 3, an overrunning clutch 7 engaging with a starter output shaft 6 of this planetary reduction assembly 5, a pinion 8 integrated with the overrunning clutch 7 and slidably disposed on the starter output shaft 6, an electromagnetic switch 9 for controlling an electric supply to the starter motor 3 and for pushing the pinion 8 together with the overrunning clutch 7 towards a ring gear 14 of an engine by means of a shift lever 10, and the like.
The starter motor 3 is composed of a yoke 11 formed into a cylindrical shape having a bottom portion also functioning as an outer frame and a magnetic circuit, a field coil 12 wound around this yoke 11, an armature 13 disposed within this field coil 12, a rectifier (not shown) mounted on the motor output shaft 4, which is a rotary shaft of the armature 13, a brush (not shown) disposed in sliding contact with this rectifier, and the like. Then, a rear bracket 2 is fitted around an outer circumference of a rear end of the yoke 11 and joined to the yoke 11 to thereby support the rear end of the motor rotary shaft 4. Also, a front bracket 1 is fitted around an outer circumference of a front end of the yoke 11 and joined to the yoke 11.
The planetary reduction assembly 5 is composed of a sun gear 15 formed around an outer circumference of a front end of the motor rotary shaft 4, a plurality of planetary gears 16 meshing with this sun gear 15 and an internal gear 17 meshing with each of the planetary gears 16.
In the internal gear 17, a center hole is formed in a central portion of its bottom portion, a rotation stop 19 is formed on an outer circumferential wall surface, and an inner circumferential gear portion 18 is formed into a bottomed cylinder engraved in the inner circumferential wall surface. Then, the internal gear 17 is fitted in the front bracket 1 so as to open on the rear side (on the side of the armature). At this time, the rotation stop 19 is engaged with the front bracket 1 so that the movement of the internal gear 17 in the circumferential direction is restricted.
A discoid flange portion 20 is formed integrally with a rear end of the starter output shaft 6. Then, a plurality of pins 21 are implanted concentrically at an equiangular pitch on the rear end surface of the flange portion 20. The planetary gears 16 are supported rotatably to the respective pins 21. This flange portion 20, i.e., the starter output shaft 6 is rotatably supported through a bearing 22 fitted in the center hole of the internal gear 17 fixed to the front bracket 1 so that the front end portion of the motor output shaft 4 is rotatably supported through a bearing 23 fitted in the flange portion 20. Thus, the plurality of planetary gears 16 mesh with the sun gear 15 and the inner circumferential gear portion 18 to thereby constitute a planetary reduction mechanism.
The sun gear 15 rotates together with the motor rotary shaft 4 to transmit the rotation of the motor rotary shaft 4 to each planetary gear 16. Then, each planetary gear 16 is subjected to the rotation of the sun gear 15 to revolve around the outer circumference of the sun gear 15 while rotating on its axis. The starter output shaft 6 is drivingly rotated by the revolution of the planetary gears 16.
The overrunning clutch 7 is mounted on the starter output shaft 6 so as to be able to move in the axial direction and such that the rotational motion is transmitted thereto. That is, the overrunning clutch 7 is spline-fitted onto the starter output shaft 6. The pinion 8 is joined to a front end portion of a sleeve shaft 24 which constitutes the overrunning clutch 7. The shift lever 10 is mounted rotatably about a pivot portion 10a in it s inter mediate portion with its one end being engaged with the overrunning dutch 7 and the other end being coupled to a plunger (not shown) of the electromagnetic switch 9 mounted above the starter motor 3.
The operation of the thus constructed conventional electric starter motor will now be described.
First of all, before the operation of the starter motor assembly, the shift lever 10 is located in the position shown in FIG. 4, the overrunning clutch 7 has not yet been moved and the pinion 8 is not engaged with the ring gear 14.
Under this condition, when the key switch (not shown) is closed and the electric starter motor is operated, the armature 13 is electrically biased by the electric supply from the electromagnetic switch 9 and is rotated with the biasing force of the field coil 12. The motor output shaft 4 formed integrally with the armature 13 is drivingly rotated in accordance with the rotation of the armature 13. Also, the shift lever 10 is driven by the plunger within the electromagnetic switch 9 so that it is rotated about the pivot portion 10a in the counterclockwise direction in FIG. 4. The overrunning clutch 7 is pushed by the rotation of this shift lever 10. The overrunning clutch 7 and the pinion 8 are moved forward in one piece along the starter output shaft 6 (in the right direction in FIG. 4) so that the pinion 8 is engaged with the ring gear 14.
At this time, the rotational torque outputted from the armature 13 is transmitted from the sun gear 15 of the motor output shaft 4 to the planetary gears 16. Then, the planetary gears 16 are rotated between the sun gear 15 and the inner circumferential gear portion 18 while rotating about the pins 21; that is, the planetary gears 16 revolve around the sun gear 15 while rotating on their axes. The flange portion 20 that supports the planetary gears 16 rotates at a more reduced speed than the rpm of the motor output shaft 4 by the revolution of the planetary gears 16 and transmits the reduction speed rotational output to the starter output shaft 6. Then, the starter output shaft 6 rotates the ring gear 14 (crankshaft) at a reduced rpm through the overrunning clutch 7 and the pinion 8.
In such an electric starter motor, there are some cases where the crankshaft during the driving rotation is coupled with the starter output shaft 6 so that the crankshaft is abruptly stopped or where the starter output shaft 6 during the driving rotation is abruptly coupled with the crankshaft. In such a case, the excessive rotational torque would be abruptly applied to the starter output shaft 6. Then, after the rotational torque applied to the starter output shaft 6 has been transmitted from the flange portion 20 at one end of the starter output shaft 6 to the planetary gears 16, it is transmitted through the inner circumferential gear portion 18 to the internal gear 17 and at the same time transmitted through the sun gear 15 to the motor output shaft 4.
In the conventional electric starter motor, since the internal gear 17 and the front bracket 1 are coupled together by the rotation stop 19, there is a disadvantage that the shock stress caused by the excessive rotational torque abruptly changed on the loaded side is transmitted through the planetary reduction assembly 5 to the front bracket 1 and the motor output shaft 4 so that a fragile portion of each element in the output transmission system within the electric starter motor would be damaged.